Steel strips or sheets, more particularly plated steel strips such as zinc or zinc base alloy plated steel strips are often used in automobiles, electric appliances, buildings and the like with or without coating. Before coating, they must have passed a number of steps and are kept uncoated for a sustained length of time. During the process, often rust will occur and various deposits and debris will deposit and adhere to the surface of plated steel strips, both adversely affecting the adherence of a subsequently applied paint.
Thus the plated steel strips are chromate treated as a primary measure for protecting rust formation until they are used by the users. Ordinary chromate treatment can provide a limited degree of corrosion resistance which is as low as 24 to 48 hours as examined by a salt spray test. A special chromate treatment, which is a coating chromate treatment using a chromate solution having silica sol added, can achieve an increased degree of corrosion resistance which is just 100 to 200 hours as examined by a salt spray test. This order of corrosion resistance, however, is insufficient for steel strips destined for long term service under a severe corrosive environment.
For service under a severe corrosive environment, another known approach for preventing corrosion is by subjecting the plated steel strips to phosphate treatment instead of chromate treatment and applying a paint coating of about 20 .mu.m thick. Such relatively thick coatings tend to crack or peel off during pressing or mechanical forming of the associated steel strips, resulting in a local loss of corrosion resistance. In addition, since coated strips are difficult or almost impossible to weld as by spot welding, the paint coating must be removed from welding sites. Thicker coatings, of course, consume larger amounts of paint, increasing the cost.
There is a need for surface-treated steel strips which have high corrosion resistance by themselves without paint.
In general, lubricant oil is applied to steel strips before they are press formed. Thus the process must be followed by degreasing. Therefore, there is a need for surface-treated steel strips which can be press formed without applying lubricant oil.
Conventional surface-treated steel strips are processed into commercial articles through a series of steps on the user side. During handling by the operator, the strip surface is prone to be stained as by finger prints, which will contribute to a marked drop of the commercial value. Therefore, there is a need for surface-treated steel strips which are resistant to stains as by finger prints during handling.
Several prior art approaches are known to meet these demands.
Japanese Patent Publication No. 24505/1987 discloses a dual coated chromate steel strip having improved corrosion resistance and lubricity comprising a chromate coating on a zinc-base alloy plated steel substrate and a layer thereon of a urethane-modified epoxy resin containing composite aluminum phosphate, a chromium base anti-rust pigment, and a lubricant selected from polyolefin wax, molybdenum disulfide, and silicone. The coating weight of the resin layer is from 1 to 10 g/m.sup.2.
Japanese Patent Application Kokai No. 35798/1988 discloses an organic coated steel strip having improved cationic electro-deposition capability comprising a chromate coating on a zinc-base alloy plated steel substrate and a layer thereon of a urethane-modified epoxy ester resin containing silica powder, a hydrophilic polyamide resin, and a polyethylene wax lubricant. The resin layer has a thickness of 0.3 to 5 .mu.m.
Japanese Patent Application Kokai No. 73938/1987 discloses a corrosion resistant coated laminate comprising a steel substrate having a gamma-phase monolayer of nickel-containing zinc plated thereon, a chromate coating thereon, and a coating thereon containing a base resin, iron phosphide conductive pigment, and a lubricant selected from polyolefins, carboxylate esters, and polyalkylene glycols. The resin coating has a thickness of 1 to 20 .mu.m.
These three types of surface-treated steel strip are dual coated steel strips having improved corrosion resistance and lubricity characterized by having a lubricating resinous coating containing a polyolefin lubricant on the chromate coating.
The dual coated steel strips of the above-cited patent publications were successful in press forming at low speeds of about 5 mm/sec. However, they were found to give rise to several problems in actual pressing as typified by press forming at high speeds of about 250 mm/sec. Under such severe working conditions, frictional contact of the strips with the die or punch causes the strip surface to raise its temperature to 70.degree. C. or higher, at which temperature the resinous coatings become brittle and prone to separate. Powdered resin will deposit on the die and the blank being formed, adversely affecting continuity of press working and the appearance of formed articles. These strips were also found to be unsatisfactory in deep drawability.